A fuel cell is a device which can readily convert chemical energy into electrical and heat energy by the reaction of a fuel gas with a suitable oxidant supply. In a proton exchange membrane fuel cell, for example, the fuel gas is typically hydrogen, and the oxidant supply is oxygen (or more typically ambient air). In fuel cells of this type, a membrane electrode diffusion layer assembly is provided, and which includes a solid polymer electrolyte which has opposite anode and cathode sides. Appropriate electrodes are provided on the opposite anode and cathode sides. During operation, fuel gas reacts in the presence of a catalyst which is incorporated into the electrode on the anode side to produce hydrogen ions which migrate through the solid polymer electrolyte to the opposite cathode side. Meanwhile, an oxidant supply introduced to the cathode in the presence of the catalyst reacts with the hydrogen ions in the presence of the catalyst which is incorporated into the electrode on that side to produce water and a resulting electrical output. The electrical power output is withdrawn from the fuel cell by means of current collectors which are disposed in ohmic electrical contact against the anode and cathode sides of the ion exchange membrane.
Many fuel cell designs have been provided through the years and much research and development activity has been conducted to develop a fuel cell which meets the perceived performance and cost per watt requirements of various users. Despite decades of research, fuel cells have not been widely embraced except for narrow commercial applications. While many designs have emerged, and which have operated with various degrees of success, shortcomings in some peculiar aspect of their individual designs have resulted in difficulties which have detracted from their widespread commercial acceptance and perceived usefulness.
For example, one of the perceived challenges for fuel cell designers is the reduction of contact resistance between the current collector and an adjacent gas diffusion layer which is borne by the membrane electrode diffusion layer assembly. This contact resistance is, generally speaking, inversely related to the power output of the fuel cell. Consequently, lowering the contact resistance increases the overall electrical output of the fuel cell.
Still further, fuel designers have long recognized that as a fuel gas and oxidant is supplied or directed over an active area of an ion exchange membrane which is incorporated therein, several interrelated, and competing factors may come into play, and which may vary the performance of the fuel cell. These several factors that are involved in the performance of the fuel cell and the ion exchange membrane include the relative hydration of the ion exchange membrane; the concentration of the fuel and/or oxidant; and the relative temperature of the reactants and the ion exchange membrane itself. In this regard when fuel cells are designed, particular care is taken to substantially optimize the diffusion layers which are made integral with the ion exchange membrane relative to perceived operational conditions under which the fuel cell may operate.
As might be expected, as operational conditions change, these competing factors may begin to vary across the face of the active area of the membrane electrode diffusion layer assembly. As a result, the specific characteristics of the respective diffusion layers often becomes suboptimal. For example, a once optimal degree of porosity and/or permeability and hydrophobicity at a predetermined location on the ion exchange membrane may, in view of the location where the fuel gas is introduced, become suboptimal. This is also often the condition at the bleed or exhaust area of the membrane electrode diffusion layer assembly where excess fuel gas, water, and other by products are removed from the fuel cell.
In traditional fuel cell stack designs, for example, a great deal of attention has been paid to the design of fuel flow channels in order to substantially optimize the current output across the entire active area. However, notwithstanding the attempts of the prior art, even in air-cooled, planar, fuel cell stack designs, the hydration of the membrane electrode diffusion layer assembly, and ultimately its performance, varies as the fuel, gas, and air travel across the surface of the fuel cell active area.
A fuel cell having a current collector, and other structures which address these and other perceived shortcomings in the prior art practices is the subject matter of the present application.